Unable to enable Purchase Sharing for Secondary Apple Account in Family Sharing

Hello. Long time reader, first time asker…

Yesterday I rejigged my Family Sharing setup so that my Primary Apple Account is the Organizer. I have a Secondary Apple Account that I use for Media and Purchases and it is currently part of the Family Sharing setup. I want to setup Purchase Sharing using my Secondary Apple Account however when I select that option it isn’t accepted. (For context: Purchase Sharing was enabled under my previous Family Sharing setup with the Secondary Apple Account.)

Please help me understand what I’m missing, and thanks for your help in advance.